A prerequisite of micro-circuit production with a reasonable yield is defect free masks and wafers to be used in the production process. Over the past 12 years a number of systems have been developed, and patented for the automatic inspection of optical masks and wafers. (See U.S. Pat. No. 4,247,203, 4,805,123, 4,618,938 and 4,845,558). These systems perform a comparison between two adjacent dice on a photomask, reticle or wafer. Similarly, technology has evolved to inspect a die against a CAD database. (See U.S. Pat. No. 4,926,489) These optical systems are, however, limited to optical masks because defects on X-Ray masks may not be apparent in the visible or ultraviolet spectrum. Furthermore, optical inspection is limited in its resolution capability by the fundamental diffraction limit that of course also limits optical lithography. Even with phase shift mask techniques it is expected that line widths below 0.35 microns cannot be achieved with optical lithography techniques and that X-Ray lithography will dominate for line widths smaller than that.
For the inspection of X-Ray masks and very dense wafer patterns, it is expected that scanning electron microscopy techniques will be employed. Recently companies have experimented with using conventional electron microscopes for the inspection of X-Ray masks and wafers. While these experiments have been successful in detecting defects, conventional electron microscopes are much too slow and require highly skilled operators. These two attributes make such systems not practical for use in the semiconductor industry.
The present invention also uses an in-situ plasma cleaning system that avoids sputtering. A system that does not have provision to avoid sputtering was previously disclosed for electron beam writing systems in U.S. Pat. No. 4,665,315, Method and Apparatus for In-Situ Plasma Cleaning of Electron Beam Optical System, by Lawrence F. Bacchetti, et al.